Method of treating alkaline solutions



Patented Apr. 7, 1936 UNITED STATES watts PATENT OFFIQE METHOD OF TREATING ALKALINE SOLUTIONS No Drawing. Application October 20, 1932, Serial No. 638,835

5 Claims.

This invention relates to methods of prolonging the life of alkaline solutions containing dissolved or suspended lead compounds, which solutions are used for sweetening hydrocarbon oils.

In the preparation of merchantable gasoline and kerosene from the proper distillates, it is found that the distillates usually contain sulfur compounds having disagreeable odors. These sulfur compounds are mercaptans of different boiling points which are readily removed by reaction with lead compounds. So, for example, it has been common practice to treat hydrocarbon oils containing mercaptans by agitating them with an alkaline solution of sodium plumbite, prepared by dissolving litharge in a strong caustic solution. This treatment results in the formation of lead mercaptides which, on being treated with sulfur, form organic disulfides and lead sulfides. This method of treating sour hydrocarbons is spoken of in the art as sweetening, since the undesirable mercaptans have been transformed into the corresponding innocuous disulfides.

The use of sodium plumbite as a sweetening agent has, in recent years, partly been replaced by the use of suspensions of lead sulfide in alkaline solutions. Most of these processes feature the agitation of sour hydrocarbons, that is, hydrocarbons containing mercaptan compounds with suspensions of lead sulfide in strong caustic solutions, usually in the presence of oxygen or air, whereby the mercaptans are selectively oxidized to corresponding disulfides. It has been found that when using these suspensions of lead compounds in caustic solutions, that the activity of the same can be increased by treating the suspensions separately with oxygen or air under varying conditions of temperature.

In some cases it has been found economical to treat sour hydrocarbons with sodium plumbite in caustic solution until all of the soluble lead compounds have been precipitated as the corresponding sulfides. When this condition obtains, the solution is bloWn with air or oxidized at normal or elevated temperatures whereby there is obtained an alkaline solution which can be successfully used for the transformation of mercaptans into disulfides. Such suspensions of lead sulfide in caustic solution are spoken of in the art as reactivated doctor solutions.

Upon examination of the various solutions, it has been found that after continued use they contain appreciable quantities of sodium thiosulfate. The mechanism whereby this salt is formed is unknown but, after a long study, it

has been noted that the increase in concentration of sodium thiosulfate appears to parallel the decrease in activity of the sweetening suspension.

That the presence of sodium thiosulfate in 5 considerable quantity is responsible for the decreased activity of a sweetening suspension appears to be quite reasonable, since this material is an extremely active reducing agent. Its use in reducing reactions has long been known and 0 is well established in the various chemical arts.

I have found that the thiosulfates present in reactivated doctor solutions may be effectively transformed into an inactive compound which does not interfere with the proper oxidation of 15 the mercaptans. This is elfected by heating the lead sulfide suspensions or the alkaline liquid media in which the lead sulfide is suspended to elevated temperatures out of contact with air. In accordance with my invention, I have found that when a spent or a partly spent reactivated doctor solution is heated to a temperature near or above its boiling point out of contact with air, that such a solution regains its activity. Although I am uncertain as to the reaction mech- 25 anism involved, I believe that the sodium thiosulfate, in the presence of the free caustic present in the solution, reacts to yield sodium sulfate and sodium sulfide.

As a specific example of the practice of my in- 30 vention, I give herewith the procedure followed in prolonging the life of a typical reactivated doctor solution. This solution was originally prepared by suspending 1500 lbs. of lead sulfide in bbls. of 15 Baum sodium hydroxide solution. This suspension was activated by blowing it with air at normal temperature. After becoming spent, that is to say, after its activity has been reduced to such a point that it will no longer effectively sweeten sour hydrocarbons and when it can no 40 longer be activated by blowing, it can be revivified by heating. This may be accomplished by subjecting the suspension to high temperature digestion. The temperatures necessary for the conversion of the sodium thiosulfate to sodium 45 sulfate and sodium sulfide will vary and are dependent in part upon the concentration of the sodium hydroxide present, the quantity or the percentage of sodium thiosulfate as well as any other water soluble sodium salts which may 50 contribute to the common. ion effect. These temperatures lie between about and 300 F.

Although experiments have shown that the suspensions as such may be revivified, I have found that the presence of lead sulfide is not 55 necessary for successful revivification in accordance with my invention. Therefore, it is possible to revivify spent suspensions of lead sulfide in caustic by allowing the lead sulfide to settle out, then withdrawing the supernatant liquor and subjecting it to the heating operation in the absence of air. The heated liquor may then be cooled to normal working conditions and mixed with the lead sulfide or else the separated lead sulfide may be suspended in the treated filtrate and the mixture then cooled to normal temperature.

This invention readily lends itself to continuous operation, in which case a small proportion of the reactivated doctor solution which is being used for continually treating a hydrocarbon oil, is withdrawn from the main stream of a treating suspension and is heated to an elevated temperature, say 250 F., under necessary pressure and, in the absence of air. The portion of doctor solution which is revivified in this way is then continually cooled and returned to the main stream of treating suspension.

Obviously many modifications and variations of the invention, as hereinbefore set forth, may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

I claim:

1. In the treatment of suspensions of lead sulfide in alkaline solutions which have become spent in the sweetening of sour hydrocarbons, the method which comprises separating the lead sulfide from the alkaline solutions and subjecting the latter to a temperature in the range of from 190to 300F. in the absence of air, then resuspending the lead sulphide in the revivified alkaline solutions to produce a reactified treating suspension suitable for sweetening further quantities of sour hydrocarbons.

2. In the treatment of a suspension of lead sulfide in an alkaline solution which has become partly spent in the treatment of sour hydrocarbons, the continuous method which comprises continuously withdrawing a portion of the suspension, heating it to a temperature in the range of from 190 to 300 F. in the absence of air, cooling it and returning it to the main portion of the treating suspension to condition the suspension for sweetening further quantities of sour hydrocarbons.

3. In the treatment of a suspension of lead sulfide in an alkaline solution which has become partly spent in the treatment of sour hydrocarbons, the continuous method which comprises continuously separating a portion of the alkaline liquor from the suspension, heating the liquor to a temperature in the range of from 190 to 300 F. in the absence of air, cooling it and returning it to the treating suspension whereby the suspension is reactivated and it is conditioned for sweetening further quantities of sour hydrocarbons.

4. In the treatment of suspensions of lead sulfide in alkaline solutions which have become spent in the sweetening of sour hydrocarbons and which contain caustic soda and sodium thiosulfate, the method which comprises separating the lead sulfide from the alkaline solutions and subjecting the latter in the absence of air to a temperature at which the sodium thiosulfate is decomposed, then re-suspending the lead sulfide in the revivified alkaline solutions to produce a reactified treating suspension suitable for sweetening further quantities of sour hydrocarbons.

5. The method of revivifying suspensions of lead sulfide in alkaline solutions which have become spent in sweetening hydrocarbons and which contain sodium hydroxide and sodium thio- I sulfate but which are substantially free from sodium plumbite, which comprises withdrawing the suspension from contact with oil being sweetened, heating the suspension in the absence of air to a temperature in the range of 190 to 300 F. and then recirculating the suspension to contact with sour oil for eifecting further sweeten- CHARLES F. TEICHMANN. 

